1. Field of the Invention
The present invention relates to a variable phase drive mechanism, also referred to as a phase change mechanism or still more simply as a phaser.
2. Description of the Related Art
It is well known to use a phaser in an internal combustion engine to vary the phase of the camshaft(s) in relation that of the crankshaft so as to allow the opening and closing times of the inlet and/or the exhaust valves within the combustion cycle to be varied. Such phasers have a drive member coupled for rotation with the engine crankshaft, one or more driven members each coupled for rotation with the or a respective camshaft and means for rotating the drive member relative to the driven member(s).
Different types of phasers have been proposed in the prior art which are actuated mechanically and electrically but the present invention is concerned only with phasers, for example vane-type phasers, that rely on the presence of a hydraulic pressure, such as the engine oil pressure, to be able to effect a relative rotation between the drive and driven members.
A problem is encountered with such phasers when the required hydraulic pressure is low, as happens during engine start-up. Under such conditions, there may occur high levels of angular movement of the camshaft(s) because the phaser oil supply pressure is not sufficiently high to control the phase.
To prevent such uncontrolled phase fluctuation of the phaser at times when the hydraulic pressure required for actuation of the phaser is below a predetermined value, it is known, for example from US2003/0196621, to provide a phaser with a spring biased locking pin for directly coupling the drive and driven members of the phaser for rotation with one another. The pin is hydraulically retracting when the hydraulic pressure attains the predetermined value to allow normal operation of the phaser.
EP 1234954 describes a vane-type phaser which allows the phase of two camshafts to be varied relative to the crankshaft. In such a twin phaser, it would in principle be possible to provide two separate locking pins, each for locking the drive member relative to a respective one of the two driven members but in practice there is not sufficient space to accommodate two locking pins.